1. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Genetics is the science of heredity These black Labrador puppies are purebred— their parents and grandparents were black Labs with very similar genetic makeups

2. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The parents of these puppies were a mixture of different breeds. The behavior and appearance of these puppies is more varied as a result of their diverse genetic inheritance

3. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Until the 20th century, many biologists erroneously believed that –characteristics acquired during lifetime could be passed on to the next generation (Lamarck) –characteristics of both parents blended in their offspring (back x white = gray)

4. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants

5. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Mendel crossed pure breeding pea plants that differed in certain characteristics and traced the traits from generation to generation: P1 = paternal F1 = first filial

6. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The F1 generation did not show blending The F2 (second filial) generation showed the reemergence of the parental trait (white) Mendel proposed that there were two genes for each trait and that they segregate from one another. Sounds like meiosis! Mendel’s First Law: Segregation

7. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Mendel studied seven pure breeding pea characteristics

8. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A sperm or egg carries only one allele of each pair, Alleles separate (segregate) when gametes form –This process describes Mendel’s law of segregation –Alleles can be dominant or recessive Phenotype = appearance Genotype = genes that cause the appearance

9. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Homologous chromosomes bear the two alleles for each characteristic

10. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s Second Law: Independent assortment By observing the inheritance of two characteristics at once, Mendel found that any one pair of alleles segregate independently of any other pair of alleles during gamete formation X

11. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

12. Independent assortment of two genes in the Labrador retriever When the dominant phenotype is expressed, there is ambiguity about the genotype.

13. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance follows the rules of probability The rule of multiplication can be used to determine the probability of certain events occurring

14. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings White belt – dominant traitSyndactyly – dominant trait Dihybrid cross, monogenic, independent traits

15. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The inheritance of monogenic human traits follows Mendel’s Laws and the rules of probability If you possess a dominant trait, so does one of your biological parents. If you possess a recessive trait, your biological parents may or may not. Genetic traits in humans can be tracked through family pedigrees

16. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Family pedigrees are used to determine patterns of inheritance and individual genotypes: deafness is a recessive trait. dd D_

17. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The offspring of a testcross can reveal the genotype of an individual when it is unknown

18. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete or partial dominance

19. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Most genetic diseases (cystic fibrosis, sickle-cell disease) are caused by recessive alleles. The heterozygous parent is a carrier.

20. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

21. A few genetic diseases (achondroplasia, Huntington’s disease) are caused by dominant alleles D_ achondroplastic dwarf

22. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings VARIATIONS ON MENDEL’S PRINCIPLES The relationship of genotype to phenotype may be more complex –Incomplete dominance –Codominance –Pleiotrophy –Polygenic traits –Sex linkage –Linkage –Epistasis

23. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Incomplete dominance in human familial hypercholesterolemia

24. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In a population, multiple alleles often exist for a characteristic –The three alleles for ABO blood type in humans is an example I A gene codes for I A protein of RBC surface I B gene codes for I B protein of RBC surface i gene codes for nothing

25. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –The alleles for A and B blood types are codominant meaning that both are simultaneously expressed in the phenotype

26. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A single gene may affect phenotype in many ways. This is called pleiotropy

27. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A single characteristic may be influenced by many genes: polygenic traits Incomplete dominance

28. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gender determination

29. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

30. In many organisms, the X chromosome carries many genes unrelated to sex. Any gene linked to the X chromosome (Z in birds) is said to be sex- linked. Males are said to be hemizygous.

31. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

34. Sex-linked disorders affect mostly males Red-green color blindness is a result of a lack of red receptors in the retina 45 8

35. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Hemophilia

36. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

37. Labrador Coat Colors Produced by two independently assorting genes for coat color that interact with one another: epistasis Black = B_Chocolate = bbYellow = B_ or bb with epistatic ee

38. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Yellow pigmentBrown pigmentBlack pigment E_ B_ E_ bb ee

39. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Recessive epistasis Variation of 9:3:3:1 ratio in F 2 due to masking of alleles by recessive epistasis (ee) Gene 1Gene 2 Gene 1Gene 2 B_ = blackE_ = no color effect bb = brownee = yellow ee double recessive overides ee double recessive overides all other coat colors regardless of dominance! Recessive epistasis

40. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Some genes are linked. They are inherited together because they reside close together on the same chromosome. Linkage is a violation of Mendel’s Second Law http://www.fspublishers.org/web site/images1/51435_..pdf

41. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Bateson and Punnett, 1908 Unlinked? Linked? Or something in between?

42. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Tetrad (homologous pair of chromosomes in synapsis) Breakage of homologous chromatids Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types 1 2 3 4 Coat-color genes Eye-color genes How crossing over leads to genetic recombination

43. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

44. Crossing over is more likely to occur between genes that are farther apart. Recombination frequencies, therefore, can be used to map the relative positions of genes on chromosomes

45. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A partial genetic map of a fruit fly chromosome

46. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chromosome behavior during meiosis accounts for Mendel’s Laws

47. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chromosome behavior during meiosis accounts for Mendel’s Laws